The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the acquisition of cardiac gated images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal emitted by the excited spins may be received after the excitation signal B.sub.1 is terminated and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When applied to cardiac imaging, for example, a complete scan from which a series of images showing the heart at different phases of its cycle can be acquired in a single breath-hold.
Whereas the more conventional pulse sequences have repetition times TR which are much greater than the spin-spin relaxation constant T.sub.2 so that the magnetization has time to relax between the phase coherent excitation pulses in successive sequences, the fast pulse sequences have a repetition time TR which is less than T.sub.2 and which drives the transverse magnetization into a steady-state of equilibrium. Such techniques are referred to as steady-state free precision (SSFP) techniques and they are characterized by a cyclic pattern of transverse magnetization in which the resulting NMR signal refocuses to produce an echo signal.
One such SSFP pulse sequence is called gradient refocused acquired steady-state (GRASS) and it utilizes a readout gradient G.sub.x to shift the peak in the NMR echo signal that is produced after each RF excitation pulse toward the center of the pulse sequence. This pulse sequence is shown in FIG. 3, where the NMR signal is a gradient recalled echo that is induced by the readout gradient G.sub.x. In two-dimensional imaging, a slice selection gradient pulse is produced by the gradient G.sub.z and is immediately refocused in the well-known manner. A phase encoding gradient pulse G.sub.y is produced shortly thereafter to position encode the acquired NMR data, and to preserve the steady-state equilibrium, the effects of the phase encoding gradient pulse are nullified by a corresponding G.sub.y rewinder gradient pulse after the NMR signal has been acquired and before the next pulse sequence begins as described in U.S. Pat. No. 4,665,365.
Because SSFP sequences employ RF excitation pulses with small tip angles and the magnetization is not allowed to recover after each pulse sequence, the image contrast due to spin density is not nearly as good as with conventional pulse sequence. Consequently, other image contrast enhancement methods have been proposed which rely on the different T.sub.1 and T.sub.2 constants of tissues.. As described by A. Haase in "Snapshot Flash MRI Applications to T.sub.1, T.sub.2, and Chemical-Shift Imaging," Magnetic Resonance In Medicine, 13, 77/14 89 (1990), and D Matthaei et al in "Fast Inversion Recovery T.sub.1 Contrast and Chemical Shift Contrast In High Resolution Snapshot Flash MR Images, " Magnetic Resonance In Medicine, Vol 10, pp. 1-6, 1992, and U.S. Pat. No. 5,256,967 entitled "Fast NMR Image Acquisition With Spectrally Selective Inversion Pulses," a series of SSFP pulse sequences may be preceded by one or more preparatory RF pulses which condition the spin magnetization to provide T.sub.1 or T.sub.2 enhanced contrast images. These methods all require a considerable waiting period before acquisition of image data in order to allow the contrast to evolve.
Cardiac gated acquisitions are employed to produce images depicting the heart at different phases of the cardiac cycle. By using SSFP pulse sequences, a "group" of k-space lines, or views (e.g. 8) may be acquired during each cardiac cycle for a particular slice location. As a result, data for an image may be acquired in a succession of cardiac cycles and during a single breath-hold. Note that each group of views may be acquired from the same spatial location, in which case a series of images at the same spatial location is obtained with each image acquired at a different temporal phase of the cardiac cycle. This represents a multi-phase or cine acquisition. In addition, each group may be acquired from different spatial locations, in which case a series of images, each at a different spatial location are acquired at different temporal phases of the cardiac cycle. This second implementation represents a single-phase multi-slice acquisition where temporal coverage of the cardiac cycle is traded-off for greater spatial coverage in an acquisition.
In the case of a single-phase multi-slice acquisition, the magnetization at a given spatial location is not at dynamic equilibrium. Since one cardiac cycle is of the order of 1 second, the longitudinal magnetization has sufficient time to relax to its thermal equilibrium value between groups. As a result, during each group of SSFP pulse sequences, the magnetization does not have time to establish a steady-state condition. The resulting image contrast is then primarily determined by spin density rather than T.sub.1 , and valuable clinical information is lost.